When printing a document, a host computer first translates the document from a language that the host application understands to one that the printer understands. By proper use of the printer language, the host computer instructs the printer what to print. In a modern printing environment, there are multiple printer languages, also referred to as printer personalities, available to a user.
A printer personality, which is typically implemented as a software/firmware module, embodies the necessary instructions to properly accept incoming data in a particular printer language and convert that representation to a simple data stream used by the print engine to print a page. The printer personality is responsible for such tasks as accepting and storing downloaded fonts, selecting and deselecting printer settings, giving feedback to the host on the status of a printer job, and so forth.
Typically, a printer is implemented with the printer personality of the manufacturer. For example, Hewlett-Packard Company uses a printer personality based on its PCL.TM. printer language. More specifically, PCL is a page description language: the data stream describes what the page should look like and prints the page when it is complete.
Some printers support multiple printer personalities, allowing the user to use the same printer with a variety of applications and hardware. Examples of other printer languages include ESC/P.TM. from Seiko Epson Kabushiki Kausha, and PostScript.TM. from Adobe Systems, Inc.
Each personality uses resources of the printer to execute its job. Such resources include RAM (Random Access Memory), ROM (Read Only Memory), hard disk, I/O channels, control panel, paper trays, and print engine. The personalities may share printer resources or partition the resources in such a way that there is minimal interaction between personalities.
Conventional printers, and particularly laser printers manufactured by Hewlett-Packard Company and others, employ an object-oriented approach to building a page. This approach involves handling items that are to be printed on the page as a set of objects. The page description for PCL-based printers consists of objects combined with colors and patterns. An obvious benefit of an object-oriented page description language is that the objects do not have to be created in a position dependent order. The kinds of objects used in PCL are text, rules (rectangles), graphics (vectors and polygons), images, and patterns.
A parser uses the objects to construct a list, known as a "display list." The display list tracks a temporal order of the objects according to a first in, first out (FIFO) protocol. The display list is sent to an image processor, which may apply colors and patterns to the listed objects.
While the printed results appear as a unified image, the printer actually segments the page into horizontal bands or "strips". FIG. 1 shows a page 20 with four strips 22a-22d. Each strip has an associated display list, so that there are many display lists per page. Each display list identifies the objects that affect the associated strip. The display list entries contain information concerning how to print the objects within the strips.
Prior to the present invention, each printer personality loaded on a printer essentially required its own display list processing pipeline. Each personality was required to construct and maintain its own display list. While this separate pipeline approach has a benefit in avoiding incompatibility problems caused by switching among multiple personalities, it has several drawbacks including additional memory capacity and additional debug support to identify various bugs introduced by different display list technologies.